1. Field of the Invention
The present invention relates to a semiconductor fabricating method. More particularly, the present invention relates to a method of fabricating a dielectric layer comprising a multiple nitride layer structure.
2. Description of the Related Art
Dielectric layers are widely used in integration circuits. The quality and the structure of the dielectric layers are important factors for a integration circuit fabrication. For example, in a DRAM fabrication process, a cell comprises a transistor and a capacitor. The capacitor comprises a dielectric layer between a top electrode and a bottom electrode. The thickness and the structure of the dielectric layer used in the capacitor affect the capacity and the quality of the capacitor. The dielectric layer is used to store electrons. Electron storage capacity is related to the dielectric constant and the thickness of the dielectric layer. In addition, the time for preserving the storage electrons is affected by the quality of the dielectric layer. Thus, it is desirable to fabricate an improved dielectric layer.
In a current fabrication process of an integration circuit, the frequently used materials of the dielectric layer are oxide and nitride. Because the nitride has low dielectric constant and the oxide has a good adhesion ability, an oxide/nitride/oxide structure are commonly used in the dielectric layer of the capacitor.
FIG. 1 is a schematic, cross-sectional view of a conventional dielectric layer comprising an oxide/nitride/oxide structure.
In FIG. 1, a RCA cleaning step is performed on a polysilicon layer 100. A HF surface treatment is performed on the polysilicon layer 100. A native oxide layer is formed on the polysilicon layer 100. The native oxide layer serves as a first oxide layer 102 of the conventional dielectric layer. A nitride layer 104 is formed on the first oxide layer 102 by deposition with a mixed gas source. The mixed gas source is mixed from SiH.sub.2 Cl.sub.2 and NH.sub.3 having a ratio of 1:10. A portion of the nitride layer 104 on the surface is oxidized. A second oxidation layer 106 thus is formed on the nitride layer 104. A dielectric layer comprising an oxide/nitride/oxide structure is formed.
In the conventional dielectric layer, only one nitride layer is used. Once defects are formed in the nitride layer, or the nitride layer is damaged in a subsequent step, current leakage is likely to occur. The current leakage causes the electrons stored in the capacitor to be lost. This, in turn, reduces the electron-preserving time for the capacitor. Therefore, the electron-refreshing step must be performed more frequently. However, frequently performing the electron-refreshing step decreases the operation speed. Thus, problems are still encountered.